1) A data copy app. I often need to copy data from one place to another (eg. copy data from MS Access to a MySQL server, export data from PostgreSQL to CSV, fetch data from some REST API and insert it into PostgreSQL). There tools for some of those jobs (eg. CSV import / export from PostgreSQL is solved). But there are a lot of jobs where I end up needing to write a custom Python script just to batch import some data from somewhere. It would be lovely if I just had a GUI app that was built for moving data from A to B.
2) A simple reporting tool for PostgreSQL. Often I just want to create a quick report from some data in a database (eg. how many users have already updated to the latest version?), but I don't want to start learning something like Tableau. I wish there was something more lightweight and affordable for creating just an occasional graph or summary table.
Both of these could start out very simple (do just one little thing) and expand from there, so I think they would be suitable for a side project.
I also think those two ideas have a lot of potential to make money. It's a huge market, and there's plenty of money to be made if you focus on smaller customers or niches that are not interesing for the big players.